The present invention relates to a steering device for a vehicle to which a so-called xe2x80x98steer by electric wirexe2x80x99 system is applied.
In a steering device for a vehicle incorporating a steer by electric wire system, the movement of a steering actuator corresponding to the operation of an operating member modelled on a steering wheel is transmitted to the wheels of the vehicle in such a manner that the steering angle changes without the operating member being coupled mechanically to the wheels. In a vehicle incorporating a steer by electric wire system of this kind, in order to stabilize vehicle behaviour, it is proposed that a target yaw rate corresponding to the amount of operation of the operation member is calculated, and the steering actuator is controlled in such a manner that this target yaw rate coincides with the actual yaw rate. Moreover, it has also been proposed that, in cases where the coefficient of friction between the road surface and the wheels of the vehicle is reduced due to freezing of the road surface, or the like, in order to prevent the vehicle behavior from becoming unstable, the braking force and driving force of the vehicle are controlled so as to eliminate the deviation between the actual yaw rate and the target yaw rate calculated in accordance with the amount of operation of the operating member and vehicle speed.
In cases where the friction of coefficient between the road surface and the wheels of the vehicle is reduced, there is a possibility that a saturated state occurs, the yaw rate of the vehicle never reaches the target yaw rate, the steering angle diverges, and the vehicle behaviour becomes unstable. Furthermore, if the control of the steering actuator interferes with the control of the braking force and driving force, it becomes impossible to stabilize the behaviour of the vehicle.
It is an object of the present invention to provide a steering device for a vehicle which is capable of resolving the aforementioned problems.
The steering device for a vehicle according to the present invention comprises: an operating member; a steering actuator driven by operation of the operating member; means for transmitting the movement of the steering actuator to the vehicle wheels, in such a manner that the steering angle changes in accordance with the movement of the steering actuator, without the operating member being coupled mechanically to the vehicle wheels; means for detecting the amount of operation of the operating member; means for detecting the vehicle speed; means for detecting the lateral acceleration of the vehicle; means for detecting the yaw rate of the vehicle; means for calculating a first steering angle set value corresponding to the detected amount of operation and vehicle speed, on the basis of a stored relationship between the amount of operation, the vehicle speed, and the first steering angle set value; means for calculating a first target yaw rate corresponding to the calculated first steering angle set value, and the detected vehicle speed, on the basis of a stored relationship between the first steering angle set value, the vehicle speed, and the first target yaw rate; means for calculating a second target yaw rate corresponding to the detected lateral acceleration and vehicle speed, on the basis of a stored relationship between the lateral acceleration, the vehicle speed, and the second target yaw rate; means for comparing the absolute value of the calculated first target yaw rate with the absolute value of the second target yaw rate; means for calculating a second steering angle set value corresponding to the difference between the detected yaw rate and a target yaw rate, as which the first target yaw rate or the second target yaw rate whichever has the smaller absolute value is taken, on the basis of a stored relationship between the difference and second steering angle set value; and means for controlling said steering actuator in such a manner that the steering angle corresponds to a target steering angle, that is the sum of the calculated first steering angle set value and second steering angle set value.
According to the constitution of the present invention, in cases where there is no change in the yaw rate even when the steering wheel is operated due to reduction in the coefficient of friction between the road surface and the vehicle wheels, the absolute value of the first target yaw rate corresponding to the amount of operation of the operating member and the vehicle speed becomes greater than that of the second target yaw rate corresponding to the detected lateral acceleration and vehicle speed. In this situation, the second steering angle set value is calculated in response to the difference between the detected yaw rate and the second target yaw rate, of which absolute value is smaller than that of the first target yaw rate. In other words, this second steering angle set value reflects the actual behaviour of the vehicle. Therefore, by taking the target steering angle as the sum of this second steering angle set value and the first steering angle set value corresponding to the amount of operation of the operating member and vehicle speed, it is possible to prevent divergence of the steering angle and hence to achieve stabilization of the vehicle behaviour.
Furthermore, in cases where there is no reduction in the coefficient of friction between the road surface and the vehicle wheels and hence the absolute value of the first target yaw rate is smaller than that of the second target yaw rate, the second steering angle set value is calculated in accordance with the differential between the first target yaw rate and the detected yaw rate, and consequently it reflects the amount of operation of the operating member operated by the driver and the vehicle speed. Therefore, by taking the target steering angle as the sum of this second steering angle set value and the first steering angle set value corresponding to the amount of operation of the operating member and vehicle speed, it is possible to cause the vehicle behaviour to change in an optimum manner in accordance with the amount of operation of the operating member and vehicle speed.
Desirably, the present invention comprises means for controlling at least one of the braking force of the vehicle wheels and the driving force of the vehicle wheels, so as to eliminate the difference between the detected yaw rate and the target yaw rate, as which the first target yaw rate or the second target yaw rate whichever has the smaller absolute value is taken.
Thereby, both the steering angle and at least one of the braking force and driving force of the vehicle are controlled so as to eliminate the deviation between the detected yaw rate and the target yaw rate, as which the first target yaw rate or the second target yaw rate whichever has the smaller absolute value is taken. Therefore, it is possible to prevent mutual interference between the control of the steering angle and the control of the braking force and/or driving force.
Desirably, the present invention comprises means for controlling the output of the engine generating driving force for the vehicle, so as to eliminate the difference between the first target yaw rate and the detected yaw rate.
In cases where the vehicle behavior is liable to become unstable due to reduction in the coefficient of friction between the road surface and the wheels of the vehicle, since the absolute value of the first target yaw rate is greater than that of the second target yaw rate, then the differential between the first target yaw rate and the detected yaw rate is greater than the differential between the second target yaw rate and the detected yaw rate. By controlling the engine output so as to eliminate this larger differential, the amount of suppression of the engine output is increased. Thereby, if the coefficient of friction between the road surface and the vehicle wheels is reduced, the amount of suppression of the engine output is increased, thereby making it possible to stabilize the behaviour of the vehicle.
According to the present invention, in a vehicle incorporating a steer by electric wire system, in cases where the coefficient of friction between the road surface and the vehicle wheels is reduced, it is possible to control the steering actuator in such a manner that there is no divergence in the steering angle, without interference between the control of the steering actuator and the control of the braking force and/or driving force of the wheels. Moreover, by controlling the output of the engine generating driving power for the vehicle, it is possible to provide a steering device for a vehicle whereby stabilization of vehicle behaviour can be achieved in an efficient manner.